ISOTOPIC EVIDENCE FOR THE SOURCE OF SULFUR IN THE UPPER FREEPORT COALBED (WEST-CENTRAL PENNSYLVANIA, USA)

Sulfur isotopic variations were used to determine the sources of sulfu r in a medium-sulfur coal (approximately 2 wt% S) that lacked marine i nfluence, which is often cited as a major source of sulfur in coal. Va riations in the amount and isotopic composition of the organic and pyr itic sulfur among the coal-bed facies of the Upper Freeport coal bed ( Middle Pennsylvanian) are attributed to differential incorporation of syngenetic and epigenetic sulfur. These variations reflect varying env ironmental conditions during paleopeat formation, porosity and permeab ility, and the availability of sulfate and iron. A large increase in t he abundance and sulfur isotopic value of pyritic sulfur in the upper facies of the coal bed, in proximity to the overlying lacustrine shale , corresponds to an increase in massive and irregular forms of pyrite. This relationship is attributed to at least two stages of epigenetic pyrite incorporation. An early stage of epigenetic pyrite, moderately enriched in S-34, apparently formed during the fluvial-lacustrine peri od immediately following peat accumulation. A late stage of epigenetic pyrite, highly enriched in S-34, formed after compaction of the origi nal peat as cleat- and fracture-filling pyrite; this sulfur was probab ly derived from bacterial reduction of dissolved sulfate in groundwate r. Isotopic mass-balance calculations indicate that this late-stage ep igenetic pyrite may account for up to approximately 50% of the pyritic sulfur in some upper facies of the coal bed. these results suggest th at most of the pyritic sulfur in the Upper Freeport coal bed may be ep igenetic, incorporated either soon after peat accumulation or later du ring coalification.